Contrary Temperature Trend Stalls Upgraded Climate Model’s Debut

Model builders investigate a puzzling malfunction in what’s expected to be the improved next version of the popular Community Earth System Model.

Screenshot from a high-resolution simulation depicting surface wind speeds using CESM. The warmer the color is, the faster the wind speeds are; the deepest reds indicate winds blowing at or greater than 30 meters per second, whereas the deepest blues are slow-moving or still air masses. Credit: UCAR

Many researchers around the world use a digital representation of Earth’s land, water, and air known as the Community Earth System Model (CESM) to mimic and predict the evolution of our planet’s climate. To enable those climate modelers to simulate global climatic behavior with ever greater precision and fidelity, scientists who work to improve CESM expected to unveil the results of the model’s next big version, CESM2, at a workshop in Boulder, Colo., last month.

“We feel really, really good about the new model.”It didn’t happen. Instead, the workshop transformed from a debut to an effort by the model’s developers to figure out why the enhanced model fails to replicate an important temperature trend during a 20-year stretch in the middle of the last century. Although observations show that global temperatures steadily rose during that mid-20th century period (as they continue to do today), the revamped model finds that they edged downward by some 0.2°C or 0.3°C during that perplexing 2-decade period.

“We feel really, really good about the new model,” said Jean-François Lamarque, an atmospheric chemist at the National Center for Atmospheric Research (NCAR) in Boulder and the chief scientist behind CESM. “Which is also the really frustrating part—it feels really good, but then there is something keeping us from giving it to people in the research community.”

Aerosol Effects Surprise

The discrepancy arose when the model’s developers added a mathematical function that calculates aerosols’ effects on cloud formation, said Richard Neale, an NCAR project scientist who works on atmospheric components of CESM. The added function, he said, “accounts for things like emissions off the coast of Southeast Asia” and in mainland China. For example, “Beijing often has these ‘brownouts’ where you can’t see farther than 3 feet in front of your face, and those emissions impact clouds.” Those emissions include aerosols like sulfate ions, which form from sulfur dioxide emitted from coal-fired power plants and other industrial processes, he explained. Sulfate particles can take up water and cause clouds to swell and, as a result, scatter incoming solar radiation. Some of this solar energy radiates back into space, which can cool the planet.

When the CESM team activated its simulation of the aerosol effect, “things went wrong,” Lamarque said. Neale suspects that the error may lie not within the climate model’s software but in the estimates of aerosol emissions for the past 160 years. The measurements of emissions levels in the data sets may be too high, he speculated.

Software Bug or Flawed Observation Data?

Working through such mismatches is a normal part of how climate models evolve.“It is yet to be determined whether a fix to the aerosol emission data sets will remedy the problem…or whether the cloud-aerosol mechanisms in the model could be too strong,” Neale said. Still, Lamarque added, working through such mismatches is a normal part of how climate models evolve: “We do simulations, and when some things look bad, we’re like ‘okay, for the next version, let’s fix it,’” he said.

The new release date for CESM2 is August of this year, at the earliest. When CESM2 launches, it will be a significant improvement over previous versions, Neale asserted. For instance, the revised model depicts precipitation more realistically than does the current version. In today’s CESM, rain or snow falls from a cloud to the ground instantaneously. That’s “not realistic, of course, as rain and snow do not fall at infinite speeds to the ground,” Neale said. The new version of the model accounts for how long it actually takes rain and snow to fall from the cloud to the ground.

When the problem with the cloud-aerosol effect gets straightened out, the model will better reflect the physics of clouds as seen in the real world, Neale predicted. Cloud-aerosol interactions are “the big advancement that we put into the model over the past few years,” he noted.

As for the erroneous simulations plaguing that major addition? “We’re trying to get to the bottom of it,” Lamarque said.

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